Advancing gelation front cure technique

ABSTRACT

A method for curing resin in the fabrication of a composite material is described comprising enclosing the composite material within a flexible vacuum enclosure, and substantially simultaneously pressing the material within the enclosure between first and second platens of a press to preselected pressure, evacuating the enclosure, selectively heating one of the platens according to a preselected scheme to preselected cure temperature to cure the resin within the material, and cooling the other platen to maintain a preselected temperature differential between the platens across the thickness of the material, whereby cure of the resin is effected in the material first near the heated platen and progresses through the thickness of the material toward the cooler platen as the material is heated to the cure temperature.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or forthe Governmwnt of the United States for all governmental purposeswithout the payment of any royalty.

BACKGROUND OF THE INVENTION

The present invention relates generally to fabrication and curingmethods for composite materials, and more particularly to a method forcontrolling resin cure in the fabrication of composites.

In existing fabrication processes for composite materials, resin cureoften requires removal of volatile constituents through partially curedresin as a result of the curing process proceeding generally inwardly ofthe material. The resultant cured composite structure is thereforefrequently characterized by an unacceptably high void volume and degreeof microcracking.

The invention has substantially reduced in critical importancepreviously existing problems with curing processes for compositematerials, and finds basis in part in the recognition that the mechanismand rate of removal of volatile constituents from the curing processchanges upon gelation of the resin. By maintaining and controlling apreselected temperature differential across the thickness of a compositelaminate during the curing process, and allowing one side of thelaminate to breathe, a gel front is generated in the laminate whichadvances with cure of the resin from a heated nonbreathing side of thelaminate to the cooled breathing side. Volatile constituents from thecuring process are transported toward the breathing side of the laminateahead of the front through a less viscous, substantially uncured portionof the laminate which facilitates removal of the volatile constituents,avoids stresses internal of the laminate structure which would otherwiseresult in cracking, and substantially eliminates void formationresulting from trapped residual volatiles. The invention has particularapplication in fabricating thick or complex composite structuresutilizing a condensation curing polymer.

It is therefore a principal object of the invention to provide animproved composite fabrication method.

It is another object of the invention to provide a method forcontrolling resin cure in the fabrication of composite materials.

These and other objects of the invention will become apparent as thedetailed description of representative embodiments proceeds.

SUMMARY OF THE INVENTION

In accordance with the foregoing principles and objects of theinvention, a method for curing resin in the fabrication of a compositematerial is described comprising enclosing the composite material withina flexible vacuum enclosure, and substantially simultaneously pressingthe material within the enclosure between first and second platens of apress to preselected pressure, evacuating the enclosure, selectivelyheating one of the platens according to a preselected scheme topreselected cure temperature to cure the resin within the material, andcooling the other platen to maintain a preselected temperaturedifferential between the platens across the thickness of the material,whereby cure of the resin is effected in the material first near theheated platen and progresses through the thickness of the materialtoward the cooler platen as the material is heated to the curetemperature.

DESCRIPTION OF THE DRAWINGS

The invention will be clearly understood from the following detaileddescription of representative embodiments thereof read in conjunctionwith the accompanying drawings wherein:

FIG. 1 is a schematic of a hydraulic press setup useful in practicingthe advancing gel front method of the invention;

FIG. 2 is a graph of advancing gel front temperature differential withtime for a sample of FM 5064, a phenolic resin impregnated graphitelaminate prepreg;

FIGS. 3A, 3B and 3C present temperature gradient profiles in the sampleof FIG. 2 at three temperature plateaus during cure; and

FIG. 4 is a schematic of an autoclave setup useful in practicing theadvancing gel front method of the invention.

DETAILED DESCRIPTION

Referring now to FIG. 1 of the drawings, shown therein is a schematic ofa hydraulic press setup useful in practicing the advancing gel frontmethod of the invention. Hydraulic press 11 includes top platen 11t andbottom platen 11b having connected thereto means for controlled heatinqor cooling of the platens by heater 12 and coolant source 13 connectedto platens 11b,11t. Heater 12 may be connected in conventional fashionto both platens 11b,11t for selective heating of one or both of them;top platen 11t may be selectively cooled as by cooled water circulation.Press 11 may otherwise be substantially conventional with a capacity ofup to about 12 tons, cooling capacity of one platen down to about 20°C., and heating capacity up to about 200° C.

Laminate sample 15 to be cured according to the method of the inventionis placed between the platens 11b,11t of press 11 within a flexiblevacuum enclosure 17 substantially as shown in FIG. 1. In the practice ofthe invention, sample 13 may comprise thermally cured resin formulationsof substantially any type, the method being particularly applicable tomonitoring cure of condensation polymers in composite sheet material offrom about 0.635 to 2.54 mm in thickness and cured at temperatures of upto about 163° C. Enclosure 17 is supported on a steel plate 18 inthermal contact with platen 11b for conducting heat from platen 11b intoenclosure 17 to sample 15. Vacuum source 19 is operatively connected toenclosure 17 through vacuu port 20 and valve 21 for selective evacuationof enclosure 17. Flexible peripheral seal 23 between enclosure 17 andplate 18 may be included to seal enclosure 17 under the influence ofvacuum source 19.

Within enclosure 17, sample 15 is sandwiched between a pair of peelplies 25 of porous Teflon or similar material providing means to readilyseparate sample 15 for examination following cure as described below.Sample 15 and peel plies 25 ar sandwiched between bleeder cloths 27 oneof which is in laminar contact with breather mat 29 for facilitatingremoval of volatile constituents and otherwise for evacuating enclosure17. Sheet 30 of Teflon or the like may be included between the assemblyof 15,25,27 and plate 18 in order to absorb resin flow and preventbonding of bleeding materials.

The mechanism and rate of removal of volatile constituents changes upongelation as laminate sample 15 cures and, therefore, maintenance andcontrol of a suitable temperature differential across the thickness ofsample 15 according to invention, promotes controlled gelation acrossthe thickness of sample 15 such that the heated (nonbreathing) side ofsample 15 cures first, and a gel front is caused to proceed across thethickness of sample 15. Volatile constituents generated during thecuring process are transported ahead of the gel front toward the cooled(breathing) side of sample 15 through a lesser cured, less viscousportion of the composite structure as the gel front advances.

Experiments were conducted in demonstration of the method of theinvention on numerous samples of FM 5064, a laminate prepreg of phenolicresin impregnated graphite (U.S. Polymeric Corp), although it is clearthat the method described herein is adaptable to the cure of othercomposite laminates as would occur to one with skill in the field of theinvention guided by these teachings. Samples 15 of FM 5064 consisted of30 plies in an overall laminate thickness of about 8.5 mm. It was firstdetermined what the nature and extent is of the temperature gradientwhich may be maintained across the thickness of laminate sample 15 to becured in the arrangement of FIG. 1. This was performed by controllablyheating bottom platen 11b and cooling top platen 11t. Six thermocouples31 were placed at six ply intervals within sample 15 substantially asshown in FIG. 1 to monitor the temperature at selected points across thethickness of sample 15. For optimum cure of most composite materialscomprising sample 15, a pressure of about 150 to 250 psi in press 11with a reduced pressure of about 25 in. Hg within enclosure 17 aresufficient.

Referring now to FIG. 2, shown therein are the thermocouple data ofadvancing gel front temperature differential with time for a 30-plysample of FM 5064 examined in the hydraulic press setup of FIG. 1 withthermocouples placed within the structure of sample 15 as justdescribed. Bottom platen 11b was heated substantially as indicated bythe time temperature scheme suggested in FIG. 2 under about 200 psipressure and enclosure 17 under evacuation. Platen 11t was cooled by 20°C. circulating water. Plots 33-38 on FIG. 2 represent observedtemperatures at respective thermocouple locations at the bottom ofsample 15. at 6, 12, 18 and 24 ply intervals measured upwardly throughsample 15, and at the top of sample 15. The data indicated a 23°-33° C.differential at each of three temperature plateaus 40,41,42 of theheating program. After 21 hours, both platens 11b,11t were heated to174° C. Once the temperature was uniform at the thermocouple locations,sample 15 was allowed to cool under pressure and vacuum to 52° C. Thetemperature gradient profiles 40a,41a,42a within sample 15 testedaccording to the data shown in FIG. 2 are shown in FIGS. 3A, B, C forrespective plateaus 40,41,42. The data of FIGS. 3A, B, C indicate that alarger temperature gradient may be maintained at the higher temperatures(viz., plateaus 41,42). The temperature gradient was observed to benearly linear across the thickness of sample 15.

Numerous samples of FM 5064 were cured in the equipment of FIG. 1 forexamination of microstructure to test the advancing cure front method ofthe invention. Thorough examination by optical microscopy of themicrostructure of each sample following cure revealed an absence ofmicrocracks and a minimum of microvoid and agglomerated filler regionsas compared to control samples cured conventionally by the standardbleeder cloth/vacuum bag method.

For comparative purposes, a further sample 15' of FM 5064, identical insize and thickness to samples 15 above, was cured in the system of FIG.1 as just described except that the temperature program followed themedian temperature spread seen by samples 15 and no temperaturedifferential was maintained across sample 15'; all other pressure andvacuum parameters were the same as for samples 15. Sample 15' exhibitedvery little cure stress microcracking on a polished surface taken fromthe center thereof, but a relatively large amount of voids were found atabout the 7-ply and 23-ply levels, which indicate that the advancing gelfront method of the invention results in significant reduction of thenumber and size of voids in cured laminate samples in addition to areduction of stress microcracking. The advancing gel front process whichcharacterizes the invention apparently promotes a thermodynamicallycontrolled polymerization within a curing laminate which favors linearchain growth; a higher (as compared to conventionally cured laminate)molecular weight is likely achieved before gelation. As a result, muchshrinkage which occurs after gelation in laminates cured conventionally.generally occurs in laminates cured according to the invention prior togelation when stress relaxation by molecular translation is easilyachieved. Referring now to FIG. 4, shown therein is a representativeautoclave 50 setup useful for curing laminates according to theadvancing gel front method of the invention. Laminate sample 51 of FM5064 with peel plies is disposed between a plates 53,55 of aluminum orother good conductor. plate 53 is controllably cooled by coolant source57 of circulating ethylene glycol/water mixture or the like operativelyconnected to plate 53. plate 55 is controllably heated by (resistance)heater 59. Flexible vacuum enclosure 61 is sealed against plate 55 byperipheral seal 63, enclosing cooled plate 53. and is operativelyconnected to a vacuum source (not shown in FIG. 4) in manner describedabove in reference to FIG. 1. With the FIG. 4 arrangement, a temperaturedifferential of 56° C. may be maintained across the thickness of sample51. In demonstration of the invention utilizing the system of FIG. 4 forcuring laminates, sample 51 was heated to 79° C., at which point resinwithin the laminate is viscous. Plate 53 was maintained at thistemperature while that of plate 55 was raised to 107° C.

It is therefore seen that in the cure of resin impregnated laminatecomposite structures according to the invention at temperatures in therange of from about 20° to 200° C., maintaining a temperaturedifferential across the thickness of the laminate of about 10°-30° C.during the heating program of the laminate and allowing the cooler sideof the laminate to breathe, generates a gel front which advances towardthe cooler side and promotes the removal ahead of the advancing front ofvolatile constituents. The invention may be particularly applicable tothe cure of composites of the polyimide and phenolic types.

The invention as herein described therefore provides an improved methodfor controlling resin cure in the fabrication of composite laminatematerials. It is understood that certain modifications to the inventionmay be made as might occur to one with skill in the field of theinvention within the scope of the appended claims. All embodimentscontemplated hereunder which achieve the objects of the invention havetherefore not been shown in complete detail. Other embodiments may bedeveloped without departing from the spirit of the invention or from thescope of the appended claims.

I claim:
 1. A method for curing resin in the fabrication of a compositematerial, comprising the steps of:(a) enclosing composite materialincluding resin to be cured within a flexible vacuum enclosure; (b)providing a press having a first platen and a second platen andincluding means to selectively heat said first platen and said secondplaten; and (c) substantially simultaneously:(i) pressing said compositematerial including resin within said enclosure between said first platenand said second platen of said press to preselected pressure, (ii)evacuating said enclosure containing said composite material includingresin, (iii) selectively heating said first platen and said secondplaten according to a preselected scheme to respective preselected firstand second temperatures to establish a preselected temperaturedifferential across the thickness of said composite material includingresin,the higher of said first and second temperatures being selectedsufficiently high to cure said resin first at the surface of saidcomposite material including resin adjacent the platen heated to saidhigher of said first and second temperatures and said temperaturedifferential is selected sufficiently large to promote cure of saidresin progressively across the thickness of said composite materialincluding resin toward the other platen heated to the lower of saidfirst and second temperatures.
 2. The method as recited in claim 1further comprising, following step (c) thereof, the step of heating bothsaid first platen and said second platen to said higher of said firstand second temperatures to effect complete cure of said resin.
 3. Themethod as recited in claim 1 wherein said composite material includingresin is a phenolic resin preimpregnated graphite laminate.
 4. Themethod as recited in claim 1 wherein said pressing of said compositemateial including resin is performed at a pressure of from about 150 toabout 250 psi.
 5. The method as recited in claim 3 wherein said higherof said first and second temperatures is selected in the range of fromabout 100° to about 110° C.
 6. The method as recited in claim 5 whereiknsaid preselected temperature differential is selected in the range offrom about 15° to about 30° C.